In such multilevel excavations it is necessary to delay the start of operations at a lower level until well after the excavation of an upper level has been completed in order to give the caved-in overburden and the fragmented rocks in the stope of the upper level time to subside or settle into a solid structure forming a competent roof for the next-lower stope. Such a subsidence period often lasts about one to three years, depending on local conditions and official safety requirements.
The drawbacks inherent in this conventional mode of operation are manifold. Thus, the residual coal present in the loose rock structure of the upper stope may spontaneously ignite, especially in mines endangered by firedamp. Considerable expenditures are involved in measures designed to guard against serious catastrophes which, however, cannot be definitely prevented. During this waiting period, furthermore, the main galleries of the mining area and other subterranean facilities such as pumping chambers, transformer stations, power supplies and ventilation systems must be maintained, again at considerable cost and with much effort. The delay, obviously, keeps the mine output low; moreover, the theoretical excavation rate is reduced since work underneath a caved-in stope cannot be carried out as efficiently as under virgin overburden so that the rate of excavation at the lower level is diminished by about 30 to 60%. Finally, additional safety measures are generally needed even after a long subsidence period to ensure a sufficient firmness of the roof at the lower level.
According to a prior proposal, a cutting and loading machine working on a mine face of an upper bench entrains a mat of wire netting to intercept the fragmented rock. The mat and the rock fragments serve as a supplemental roof for the next-lower level, yet this technique is not free from problems of operation and safety. Thus, the correct emplacement of the mat in the wake of the excavating machine is complicated; even with proper positioning, the mat can only lessen the impact of dropping clumps of overburden upon the underlying rock structure forming the roof of the next-lower stope but cannot densify or consolidate the rubble in the upper stope and thus does not significantly contribute to the stability of the structure. Experience has also shown that the mat will withstand only limited impact and will be torn by blocks of several tons of overburden falling upon it; this may result in serious difficulties for the operations going on at the lower level. Furthermore, the mechanical stresses and dislocations caused by this method on the floor of the upper stope could promote spontaneous ignition and might result in undetected fires smoldering under the loosely piled rock fragments.
The use of liquid bonding agents to help solidify the roof or the walls of an underground vault is also known. A composition of this type, known as shotcrete, consists of a mixture of comminuted portland cement, sand and water and can be sprayed onto a tunnel wall to fill small voids between rock fragments. Another hydraulic bonding agent, described in German printed specification No. 2,216,039, comprises granular natural anhydrite and gypsum semihydrate in a certain quantity of water, to which an activator may be added. Conventional techniques for using these compositions cannot be readily utilized for reinforcing a stope, formed during excavation of a coal bench under a previously excavated and caved-in level, to prevent its premature collapse.